A non-geostationary satellite system comprises a group or constellation of satellites that orbit the earth at altitudes other than geostationary orbit above the earth's surface. Unlike geosynchronous satellite systems that have satellites in geosynchronous orbits, i.e. orbital periods exactly the same as the rotation period of the earth and thus, are fixed in space in relation to the rotation of the earth, the non-geostationary satellites move at relatively higher speeds and therefore pass overhead from horizon to horizon.
The non-geostationary satellite systems have less propagation loss and less propagation delay than satellite systems with satellites orbiting a greater distances (e.g., geosynchronous satellite systems) due to the low orbit of the non-geostationary satellites. The lower propagation loss and delay improve voice, data, and other interactive communications. In addition, the cost of non-geostationary satellite systems is less than higher-orbit satellite systems because a non-geostationary satellite's receiving antenna does not have to be as large and powerful as a geostationary satellite's receiving antenna, and the satellite launch costs are relatively less expensive.
Because the non-geostationary satellites pass quickly overhead, they must be tracked and handed off from one satellite to the next, as the satellites move in and out of range of the earth-bound user terminals, to achieve continuous data communication. User terminals that use dish antennas or continuous transverse stub (CTS) panel antennas can track satellites and handoff from one satellite to the next. Dish and CTS panel antennas each require two antennas synchronously operating with one another to instantly switch between satellites and two motors for each antenna to mechanically move the antenna. Thus, dish and CTS panel antennas are relatively expensive because they require two antennas and four motors. The use of two antennas and four motors in dish and CTS panel antennas can also decrease the reliability of the user terminal.
User terminals that use planar phased array antennas can track and handoff from satellite to satellite electronically with one antenna and without the use of motors to mechanically move the antenna. Such planar phased array antennas, however, tend to be expensive.
Accordingly, an inexpensive user terminal is needed, which can track non-geostationary satellites and instantly handoff from one satellite to another as they pass overhead.